Large-scale or process chromatography is almost entirely carried out using columns. While membranes, monoliths and other alternatives are gradually finding acceptance in niche applications such as biopharmaceutical purification, the use of columns in chromatography continues.
Process chromatography is widely used for the purification of biopharmaceuticals such as monoclonal antibodies, interferons, growth factors and vaccines. One of the major attributes of columns used in such applications is their small bed-height to diameter ratio. Chromatographic separation processes are commonly scaled-up by increasing the column diameter while maintaining the bed height constant. Several factors make is necessary to use such columns. Firstly, the pressure drop increases with bed-height and beyond a certain point becomes a limiting factor. Also, columns with small bed-heights and larger cross-sectional areas can be operated at significantly higher flow rates than tall columns of similar bed-volumes, and are therefore more productive.
Columns are easy to pack with a stationary phase and the flow of mobile phase is axis-symmetric. Samples can therefore be conveniently distributed in a symmetric fashion over its entire cross-section, and it is easy to visualize the segregation of separated bands of solutes as they move gradually towards the outlet. Also, a circular cross-section gives the maximum bed-volume per unit conduit perimeter and this is a vital factor when designing large packed-bed devices in general.
Chromatographic resins used for bioseparation tend to be “soft” and more compressible compared to those used in other applications. Consequently, in a tall column, the sheer weight of resin could result in severe compaction in material closer to the bottom, leading to inconsistencies in separation.
The use of columns with small bed-height to diameter (i.e. axial to radial dimension) ratios gives rise to some major engineering challenges. Achieving uniform flow distribution within such columns is difficult. During sample injection, non-uniform distribution may result in distortion of the sample front within the column. Similarly, during elution, the eluent front could get distorted. Overall, these factors result in radial and axial dispersion effects which results in broad and poorly resolved eluted peaks, which ultimately affect purity, recovery and productivity of a separation process. Peak broadening also results in the dilution of the material eluted from a column which in turn could represent loss of process efficiency, due to the need for downstream concentration steps.